Ischemic heart disease typically results from an imbalance between the myocardial blood flow and the metabolic demand of the myocardium. Progressive atherosclerosis with increasing occlusion of coronary arteries leads to a reduction in coronary blood flow, which creates ischemic heart tissue. “Atherosclerosis” is a type of arteriosclerosis in which cells including smooth muscle cells and macrophages, fatty substances, cholesterol, cellular waste product, calcium and fibrin build up in the inner lining of a body vessel. “Arteriosclerosis” refers to the thickening and hardening of arteries. Blood flow can be further decreased by additional events such as changes in circulation that lead to hypoperfusion, vasospasm or thrombosis.
Myocardial infarction (MI) is one form of heart disease that results from the sudden lack of supply of oxygen and other nutrients. The lack of blood supply is a result of a closure of the coronary artery (or any other artery feeding the heart) which nourishes a particular part of the heart muscle. The cause of this event is generally attributed to arteriosclerosis in coronary vessels.
Formerly, it was believed that an MI was caused from a slow progression of closure from, for example, 95% then to 100%. However, an MI can also be a result of minor blockages where, for example, there is a rupture of the cholesterol plaque resulting in blood clotting within the artery. Thus, the flow of blood is blocked and downstream cellular damage occurs. This damage can cause irregular rhythms that can be fatal, even though the remaining muscle is strong enough to pump a sufficient amount of blood. As a result of this insult to the heart tissue, scar tissue tends to naturally form.
Various procedures, including mechanical and therapeutic agent application procedures, are known for reopening blocked arties. An example of a mechanical procedure includes balloon angioplasty with stenting, while an example of a therapeutic agent application includes administering a thrombolytic agent, such as urokinase. Such procedures do not, however, treat actual tissue damage to the heart. Other systemic drugs, such as ACE-inhibitors and Beta-blockers, may be effective in reducing cardiac load post-MI, although a significant portion of the population that experiences a major MI ultimately develop heart failure.
An important component in the progression to heart failure is remodeling of the heart due to mismatched mechanical forces between the infarcted region and the healthy tissue resulting in uneven stress and strain distribution in the left ventricle. Once an MI occurs, remodeling of the heart begins. The principle components of the remodeling event include myocyte death, edema and inflammation, followed by fibroblast infiltration and collagen deposition, and finally scar formation from extra-cellular matrix (ECM) deposition. The principle component of the scar is collagen which is non-contractile and causes strain on the heart with each beat. Non-contractility causes poor pump performance as seen by low ejection fraction (EF) and akinetic or diskinetic local wall motion. Low EF leads to high residual blood volume in the ventricle, causes additional wall stress and leads to eventual infarct expansion via scar stretching and thinning and border-zone cell apoptosis. In addition, the remote-zone thickens as a result of higher stress which impairs systolic pumping while the infarct region experiences significant thinning because mature myocytes of an adult are not regenerated. Myocyte loss is a major etiologic factor of wall thinning and chamber dilation that may ultimately lead to progression of cardiac myopathy. In other areas, remote regions experience hypertrophy (thickening) resulting in an overall enlargement of the left ventricle. This is the end result of the remodeling cascade. These changes also correlate with physiological changes that result in increase in blood pressure and worsening systolic and diastolic performance.